In the manufacture of paper, an aqueous cellulosic suspension or slurry is formed into a paper sheet. The cellulosic slurry is generally diluted to a consistency (percent dry weight of solids in the slurry) of less than 1 percent, and often below 0.5 percent, ahead of the paper machine, while the finished sheet must have less than 6 weight percent water. Hence, the dewatering aspects of papermaking are extremely important to the efficiency and cost of the manufacture.
The least costly dewatering method is drainage, and thereafter more expensive methods are used, including vacuum pressing, felt blanket blotting and pressing, evaporation and the like, and any combination of such methods. Because drainage is both the first dewatering method employed and the least expensive, improvements in the efficiency of drainage will decrease the amount of water required to be removed by other methods and improve the overall efficiency of dewatering, thereby reducing the cost thereof.
Another aspect of papermaking that is extremely important to the efficiency and cost of manufacture is the retention of furnish components on and within the fiber mat being formed during papermaking. A papermaking furnish contains particles that range in size from about the 2 to 3 millimeter size of cellulosic fibers to fillers measuring only a few microns. Within this range are cellulosic fines, mineral fillers (employed to increase opacity, brightness and other paper characteristics) and other small particles that generally, without the inclusion of one or more retention aids, would pass through the spaces (pores) between the cellulosic fibers in the fiber mat being formed.
One method of improving the retention of cellulosic fines, mineral fillers and other furnish components on the fiber mat is the use of a coagulant/flocculant system, which is added ahead of the paper machine. In such a system, a coagulant such as a low molecular weight cationic synthetic polymer or a cationic starch is first added to the furnish. The coagulant generally reduces the negative surface charges present on the particles in the furnish, particularly cellulosic fines and mineral fillers, and thereby agglomerates such particles. The coagulant is followed by the addition of a flocculent. The flocculant is generally a high molecular weight cationic or anionic synthetic polymer which bridges the particles and/or the agglomerates from one surface to another, thereby binding the particles into large agglomerates. The presence of such large agglomerates in the furnish increases retention. The agglomerates are filtered out of the water onto the fiber web, where unagglomerated particles would otherwise generally pass.
While a flocculated agglomerate generally does not interfere with the drainage of the fiber mat to the extent that would occur if the furnish were gelled or contained gelatinous material, when such flocs are filtered by the fiber web the pores thereof are reduced, thus reducing drainage efficiency. Hence, the retention is increased at the expense of a decrease in drainage.
Systems, such as those described in U.S. Pat. Nos. 4,753,710 and 4,913,775, the disclosures of which are incorporated herein by reference, have been employed to provide an improved combination of retention and dewatering. Briefly, these patents call for adding to the aqueous cellulosic papermaking suspension first a high molecular weight linear cationic polymer before shearing the suspension, followed by the addition of bentonite after shearing. The shearing is generally provided by one or more of the cleaning, mixing and pumping stages of the papermaking process. The shearing breaks down the large flocs formed by the high molecular weight polymer into microflocs, and further agglomeration then ensues with the addition of the bentonite clay particles.
Another system, disclosed in U.S. Pat. No. 4,388,150, uses the combination of cationic starch followed by colloidal silica to increase the amount of material retained on the web by charge neutralization and adsorption of smaller agglomerates.
U.S. Pat. Nos. 5,098,520 and 5,185,062, the disclosures of which are incorporated herein, describe methods of improving dewatering in a papermaking process.
Despite these prior systems, there is still a need for new processes utilizing hydrophilic dispersion polymers to improve retention and drainage performance, especially without the unwanted addition of oils and surfactants which are contained in the conventional latex polymers. As used herein, "latex" is defined to mean an inverse water-in-oil emulsion polymer. There is also a need for dispersion polymers which do not require an inverter system and can be introduced to the papermaking process using simple feeding equipment.